1. Field of the Invention
The present invention relates to a signal relay apparatus in a network system that is configured with a signal branching apparatus that branches a signal transmitted via a first network to a second network, a signal relay apparatus that adds a signal transmitted via a third network to the first network, and a node apparatus that calculates a transmission path of a signal routed through the first network, a node apparatus, a network system, a virtual-link generating method, a path calculating method, and a computer product.
2. Description of the Related Art
Recently, in a large-scale network such as a backbone network of a telecommunications carrier and a wide area network (WAN), a communication is carried out between a transmission-side terminal and a reception-side terminal via a high-speed network such as an optical communication network. As a communication system for the optical communication network, a wavelength division multiplexing (WDM), a synchronous optical network/synchronous digital hierarchy (SONET/SDH), and the like are commonly used.
When a communication is carried out via the optical communication network, it is necessary to establish a path in advance between a transmission apparatus to which the transmission-side terminal device is connected and a transmission apparatus to which the reception-side terminal device is connected. The path is a route of a signal occupying a single wavelength in the optical communication.
A transmission apparatus connected to the optical communication network notifies link information including identification information of the transmission apparatus itself and identification information of a transmission apparatus connected to the transmission apparatus itself (hereinafter, “remote transmission apparatus”) to transmission apparatuses inside and outside the network, and collects link information notified from other transmission apparatuses. Upon receiving a request for establishing a path from a terminal device connected to the transmission apparatus, the transmission apparatus establishes a path to a transmission apparatus to which a reception-side terminal device is connected, based on the collected link information.
In the notification of the link information, a routing protocol is used, and in the establishment of the path, a signaling protocol is used. For instance, in a network employing a generalized multi-protocol label switching (GMPLS), an open shortest path first-traffic engineering (OSPF-TE) is used as the routing protocol, and a resource reservation protocol-traffic engineering (RSVP-TE) is used as the signaling protocol.
However, when a signal is transmitted between the transmission apparatuses in the optical communication network, the signal may not be transmitted normally in a communication after the path has been established, due to a problem in the transmission apparatus it self, a physical factor such as loss and dispersion of the signal in an optical fiber working as a transmission line and nonlinearity of the optical fiber, and a transmission distance.
To cope with the problem, a method has been developed, in which it is verified whether a signal can be exchanged between transmission apparatuses configuring an optical communication network, a virtual link is generated between apparatuses capable of exchanging the signal, and link information of the generated virtual link is notified to apparatuses outside the network, instead of link information of a link that is actually connected with an optical fiber, to hide a connection restriction inside the optical network (see, for example, Japanese Patent Application Laid-Open No. 2003-18200).
However, when the transmission apparatus configuring the optical communication network is, in the scheme of the apparatus, a transmission apparatus having a restriction of a link with a transmission apparatus outside the network, such as a reconfigurable optical add/drop multiplexer (ROADM) (hereinafter, “a ROADM device”), a transmission apparatus located at a boundary with a transmission apparatus outside the optical communication network may not be able to branch a signal to the transmission apparatus outside the network, even when the signal is transmitted from a transmission-side transmission apparatus along a path established via the virtual link.
FIG. 12 is a schematic diagram for explaining a configuration of a conventional ROADM device. A ROADM device 1 shown in FIG. 12 multiplexes five types of signals of wavelengths λ5, λ7, λ9, λ11, and λ13, and can connect three links for transmitting the signals of wavelengths λ5, λ7, and λ9 to a node apparatus outside the optical communication network via ports L, M, and N, respectively.
In other words, the ROADM device 1 is not capable of branching the signals of wavelengths λ11 and λ13. Therefore, even when a virtual path is set for transmitting the signals of wavelengths λ11 and λ13 via other ROADM device, it is not possible to branch the signals to the node apparatus outside the optical communication network.
FIG. 13 is a schematic diagram for explaining the problem with the network having a connection restriction. An optical communication network shown in FIG. 13 is configured by connecting ROADM devices 1a to 1d via an optical fiber. Node apparatuses 2a to 2d shown in the figure are node apparatuses outside the optical communication network, which are connected to the ROADM devices 1a to 1d, respectively, so that a signal is added to and branched from the optical communication network via the ROADM devices 1a to 1d. 
In the example shown in FIG. 13, the ROADM device 1a and the node apparatus 2a are connected each other via a link transmitting a signal that is transferred in the optical communication network via the wavelengths λ5 and λ9 by branching and adding the signal. The ROADM device 1b and the node apparatus 2b are connected each other via a link transmitting a signal that is transferred in the optical communication network via the wavelengths λ7 and λ9 by branching and adding the signal. The ROADM device 1d and the node apparatus 2d are connected each other via a link transmitting a signal that is transferred in the optical communication network with the wavelengths λ5 and λ7 by branching and adding the signal (An explanation will be omitted for the ROADM device 1c and the node apparatus 2c.). In addition, virtual links are generated between the ROADM devices 1a and 1d and between the ROADM devices 1b and 1d, respectively.
In this case, it is possible to set, for example, a path X for a signal transferred in the optical communication network with the wavelength λ5 between the node apparatus 2a and the node apparatus 2d. However, when it is required to set a path Y for a signal transferred in the optical communication network with the wavelength λ9 between the node apparatus 2b and the node apparatus 2d, it is not possible to set the path Y because there is no link that is capable of branching the signal between the ROADM device id and the node apparatus 2d. 
Therefore, when a path to be set is through a virtual link, it is necessary to figure out whether a ROADM device of an output port of the virtual link is capable of branching a wavelength of a signal transmitted through the path before setting the path. Although it is possible to manually input information on the wavelength that can be branched by the ROADM device at the time of setting the path, it requires lots of effort of a network administrator, which is not practical.